1. Field of the Invention
The present invention relates to a harmonic wave generating element for obtaining a coherent light having short wavelength by converting an output light from a stable light source for a high-power coherent light as a laser into a coherent light of its harmonic wave having a half or shorter wavelength of the output light.
2. Description of the Related Art
In order to obtain a harmonic wave generating element, for example, a second harmonic generating element (SHG) with a high conversion efficiency, it is important that the phase of incident light and the phase of generated SHG light are matched within the element. In the case of a bulk element, the phase matching is made by adjusting the angle of SHG crystal with respect to the optical axis using anisotropy of refractive index of SHG crystal.
Also, a method of enhancing intensity of light by use of a waveguide structure is employed in order to improve efficiency of SHG generation. Since conversion efficiency of harmonic wave generating element becomes high in proportion to intensity of light in the element, the light intensity can be enhanced by waveguide structure converging light in the element. Various methods for phase matching in a waveguide structure have been proposed. Practical ones of those methods include a method using Cherenkov radiation disclosed by P. K. Tien et al. in Appl. Phys. Lett., 17 (1970), 447 and a method for obtaining a quasi phase matching by a periodic structure disclosed by S. Somekh et al. in Appl. Phys. Lett., 21 (1972), 140, for example. The former method has a simple waveguide structure however, it requires a complicated optical for converging light since SHG light is generated radially with a fixed angle with respect to the waveguide. On the other hand, the latter method has a complicated structure and requires a long fabrication process light can easily be converged since SHG light is propagated according to a waveguide mode.
In the quasi phase matching method, a periodic structure for making a quasi phase matching and a waveguide structure for guiding light are generally provided separately from each other, which makes the structure of the element much complicated. However, J. D. Bierlein et al. in Appl. Phys. Lett., 56 (1990), 1725 reported a method in which a polarization-inverted structure is periodically formed in a KTP (KTiOPO.sub.4) crystal by ion exchange so that polarization-inverted portions form discrete waveguides. The reported method is simple and provides one of the most practical SHG elements having a quasi phase matching structure. In this method, the condition of phase matching is moderated by using an equilibrium phase matching method in which the inversion of polarization is performed in finely divided segments shorter than the coherence length to obtain a phase matching. Also, this method succeeds in fabrication of one of the efficient elements among the conventional SHG elements for the reason that reflection loss can be made small as a difference of the refractive index of the polarization-inverted portion from that of a non-inverted portion is small and the diffraction loss can be suppressed by making an interval between polarization-inverted portions sufficiently smaller than the Fresnel length.
The above-mentioned element including the combination of the equilibrium phase matching based on the periodic inversion of polarization and the discrete optical waveguides is one of the most practical SHG elements. But, such an element does not suffice for providing a truly practical element so far. In the discrete optical waveguide structure, the diffraction loss is reduced by making the interval between polarization-inverted portions smaller than the Fresnel length. In actual, however, it is not possible to suppress the losses in reflection and diffraction of light caused at interfaces or boundary surfaces to zero. In general, an SHG element is provided with 100 or more discrete structures in the direction of propagation of light. Accordingly, even small losses result in a large cumulative loss as a whole so that effectively available SHG light propagating through the waveguides is reduced, thereby making it difficult to enhance the whole SHG conversion efficiency. Furthermore, since the stray light from each discrete structure diffracts and superimposes on the output light, the output light cannot be converged into one point. In addition, since the interval between polarization-inverted portions cannot freely be selected for the need to reduce the loss, a restriction is imposed on the condition for phase matching and the degree of freedom in designing the structure is therefore low.
Another problem lies in that a very high precision is required for the a waveguide forming process. Generally, in an SHG element having a waveguide structure, a loss at the waveguide greatly depends on the surface precision or profile irregularity of a boundary between a substrate and a light confining region. Therefore, in order to reduce this loss, it is necessary to improve the precision of the boundary surface of the waveguide up to a value which is not larger than a several tenths of the wavelength. Since the existing waveguide forming process relies on a method in which a photomask pattern is transferred to a photoresist film and a portion corresponding to the pattern is subjected to ion exchange or the like, there is a problem that it is difficult to satisfy the requirements for the precision of the boundary surface.
There is a further problem associated with the waveguide structure. Namely, since the SHG conversion efficiency is proportional to the second power of the electric field strength of propagating light, the conversion efficiency is enhanced as the width of the waveguide is made smaller. However, it is difficult to form a narrow waveguide on the order of the wavelength by the actual process.
Further, a harmonic wave generating element using an optical waveguide requires the propagation of light in a single transverse mode in the waveguide in order to enhance the efficiency of conversion into harmonic wave. The width of the optical waveguide required in this case is several .mu.m which is in the order of the wavelength of light. In the case where an incident laser beam is to be converged onto an end surface of the waveguide by use of a lens, it is necessary to make the position of a convergent point coincide with the position of the end surface of the waveguide by the order of .mu.m. Therefore, a high mechanical precision and a high stability are required. However, it is difficult to make the light incident upon the waveguide with high efficiency and the coupling loss of the incident light is large.
Also, a problem common to all of SHG elements using the quasi phase matching system is that tolerance in generating condition of SHG light with high efficiency is very narrow. In general, an allowable wavelength difference is not larger than 0.1 nm in the case of usual light in the range of visible rays to near infrared rays. Accordingly, it is necessary to match the wavelength of incident laser light with the allowable wavelength of the SHG element with a high precision and to stabilize the incident laser light and it is therefore difficult to use a general and low-cost semiconductor laser the output wavelength of which changes depending upon temperature and driving current.